1. Field of the Invention
The present invention relates to an electron synchrotron accelerating apparatus, and more particularly, to an electron synchrotron accelerating apparatus for producing synchrotron orbit radiation and a method for operating the accelerating apparatus.
2. Information of the Related Art
Presently, x-ray exposure apparatuses of one type are becoming prevailing means for manufacturing semiconductors. These apparatuses can very finely transfer circuit pattern lines to wafers. In the exposure apparatuses of this type, an electron synchrotron accelerating apparatus is sometimes used as an x-ray generating apparatus, in order to generate strong x-ray with good parallelism. In the synchrotron accelerating apparatus, synchrotron orbit radiation is produced when electrons running along an orbit are deflected by magnetic fields. Soft x-ray of the orbit radiation are utilized as exposure light beams.
In the synchrotron accelerating apparatus arranged in this manner, electron beams are preliminarily accelerated and injected into an accelerating ring by means of an electron injector. These electron beams are rotated along a predetermined orbit inside the ring by deflecting magnetic fields, and are then accelerated to a rated energy level by means of a high-frequency accelerating cavity.
The electron beams are injected, in a predetermined amount (enough for a number of revolutions inside the accelerating ring) for each cycle, into the accelerating ring. This system of injection is called a multi-turn injection mode. An injection equilibrium orbit and a central orbit are defined within the accelerating ring. Each injected electron beam first rotates along the injection equilibrium orbit, while undergoing betatron oscillation around the injection equilibrium orbit. The amplitude of the betatron oscillation of the electron beam, which is great at this point of time, is subjected to gradual radiation damping. As a pulse magnetic field is reduced gradually, the injection equilibrium orbit approaches and finally overlaps the central orbit. The electron beam stably moves along the central orbit, and the amplitude of the betatron oscillation is subjected to gradual radiation damping. The time period required until the amplitude of the betatron oscillation of the electron beam reaches 1/e is called radiation damping time .tau..sub.d.
The injection of the electron beam is repeated a plurality of times for nearly each radiation damping time .tau..sub.d. More specifically, after a previously injected electron beam is damped and stabilized, a subsequent electron beam is injected into the accelerating ring. Thus, the electron beams are accumulated on the central orbit, thereby providing accumulated electron current. When the accumulated electron current is accelerated and deflected, synchrotron orbit radiation is produced. In order to increase the radiation, therefore, the flow of the accumulated electron current must be made large. The higher the frequency of the beam injection, the larger the accumulated electron current flow will be made. This system of repeated beam injection is called a multi-cycle injection mode.
The electron beams accumulated on the central orbit are changed from the phase for the stable acceleration, due to collisions with the electron beams each other, and dissipate at a certain probability. The time interval of dissipation of each electron beam is called beam lifetime .tau..sub.T. Thus, the electron beams must have been injected by the end of the beam lifetime. If radiation damping time .tau..sub.d and beam lifetime .tau..sub.T are 100 seconds and 1,000 seconds, respectively, the electron beams theoretically can be injected about ten times (1000/100 =10).
If the synchrotron accelerating apparatus is used as the exposure apparatus, it is expected to be compact. To attain this, an injector is utilized which can inject electron beams of relatively low energy of, e.g., 10 to 40 MeV into the accelerating ring.
An electromagnet for generating a deflecting magnetic field inside the accelerating ring is formed of a normal conducting magnet. The intensity of the maximum deflecting magnetic field of the normal conducting magnet is about 1.5 T. Since the field intensity is low, the circumference and deflection radius of the accelerating ring is made relatively long. If electron beams of relatively low energy of, e.g., 10 to 40 MeV are injected into the ring, radiation damping time .tau..sub.d is as long as several minutes to tens of minutes. If the accumulated electron current is 500 mA, on the other hand, beam lifetime .tau..sub.T is substantially equal to radiation damping time .tau..sub.d FIG. 1B shows the dependence of the beam lifetime and the radiation damping time on the injected electron energy, observed when the radius of the accelerating ring, the maximum deflecting magnetic field intensity, and the accumulated electron current are, for example, 3 m, 1.5 T, and 500 mA, respectively. As seen from FIG. 1B, if the injected electron energy is 30 MeV, beam lifetime .tau..sub.T is substantially equal to radiation damping time .tau..sub.d.
Therefore, the electron beam can be injected only once into the accelerating ring, that is, multi-cycle injection is impossible. Thus, if the energy of the injected electron beam is relatively low, the accumulated electron current cannot be increased to a predetermined value. This is regarded as attributable to the low intensity of the deflecting magnetic field and the long circumference of and the long deflection radius the accelerating ring.
Accordingly, the electromagnet for applying the deflecting magnetic field in the accelerating ring may be formed of a superconducting magnet, which can generate a high-intensity magnetic field despite its compactness. Since the intensity of the deflecting magnetic field is high, in this case, the circumference and the deflection radius of the ring can be made shorter than when a normal conducting magnet is used. Thus, radiation damping time .tau..sub.d can be expected to be made shorter than beam lifetime .tau..sub.T.
No researchers have studied the relationship between radiation damping time .tau..sub.d and beam lifetime .tau..sub.T in the case where a deflecting magnetic field is applied by means of a superconducting magnet, and electron beams of relatively low energy of, e.g., 10 to 40 MeV are applied to an accelerating ring. In this case, therefore, the possibility of multi-cycle injection of the electron beams has been unknown. Thus, whether or not the accumulated electron current can be increased to the predetermined value has not been determined yet.